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'Waverider' hypersonic jet is due to ride again

14 August 2012

The US Air Force experimental X-51A Waverider, an unmanned supersonic combustion ramjet-powered hypersonic flight test demonstrator, is due for another flight.

US Air Force photograph by Chad Bella

The X-51A, designed to be launched from an airborne B-52 Stratofortress bomber, is due to be released at approximately 50,000 feet over the Pacific Ocean Point Mugu Naval Air Warfare Center Sea Range. The flight test vehicle stack is approximately 25 feet long and includes a modified solid rocket booster from an Army Tactical Missile, a connecting interstage, and the X-51A cruiser. The nearly wingless cruiser is designed to ride its own shockwave, thus the nickname, Waverider.

The distinctive, shark-nosed cruiser has small controllable fins and houses the heart of the system, an SJY61 supersonic combustion ramjet or scramjet engine built by Pratt & Whitney Rocketdyne designed to burn JP-7 jet fuel. Boeing's Phantom Works performed overall air vehicle design, assembly and testing for the X-51's various component systems.

The X-51 was made primarily using standard aerospace materials such as aluminum, steel, inconel, and titanium. Some carbon/carbon composites of the leading edges of fins and cowls are used. For thermal protection, the vehicle utilizes a Boeing designed silica-based thermal protection system as well as Boeing Reusable Insulation tiles, similar to those on board the NASA Space Shuttle Orbiters. 

Four X-51As were built for the Air Force. The X-51A program is a technology demonstrator and was not designed to be a prototype for weapon system. It was designed to pave the way to future hypersonic weapons, hypersonic intelligence, surveillance and reconnaissance, and future access to space. Since scramjets are able to burn atmospheric oxygen, they don't need to carry large fuel tanks containing oxidizer like conventional rockets, and are being explored as a way to more efficiently launch payloads into orbit.

In addition to scalable scramjet propulsion, other key technologies that will be demonstrated by the X-51A include thermal protection systems materials, airframe and engine integration, and high-speed stability and control.

The X-51A represents one of the service's most significant reinvestments in hypersonic flight since the rocket-powered X-15 program which flew 50 years earlier. 

Air Force officials anticipate the X-51A program will provide a foundation of knowledge required to develop the game changing technologies needed for future access to space and hypersonic weapon applications. For example, hypersonic speeds on the order of flying 600 nautical miles in 10 minutes may provide the ability to accurately engage a long-distance target very rapidly.

The X-51A programme is a collaborative effort of the Air Force Research Laboratory and the Defense Advanced Research Projects Agency (DARPA), with industry partners The Boeing Company and Pratt & Whitney Rocketdyne. Programme management is accomplished by the Air Force Research Laboratory Propulsion Directorate at Wright-Patterson Air Force Base, Ohio.

Hypersonic flight, normally defined as beginning at Mach 5, presents unique technical challenges with heat and pressure, which make conventional turbine engines impractical. Programme officials said producing thrust with a scramjet has been compared to lighting a match in a hurricane and keeping it burning.

The Air Force currently plans to fly each X-51A on identical flight profiles. Like the X-15, the X-51A is designed to be carried aloft by a B-52 mother ship launched from the Air Force Flight Test Center at Edwards Air Force Base, California. It is released at approximately 50,000 feet over the Pacific Ocean Point Mugu Naval Air Warfare Center Sea Range. The solid rocket booster accelerates the X-51A for 30 seconds to approximately Mach 4.5, before being jettisoned. Then the cruiser's scramjet engine, remarkable because it has virtually no moving parts, ignites. The ignition sequence begins burning ethylene, transitioning over approximately 10 seconds to the same JP-7 jet fuel once used by the SR-71 Blackbird. 

Powered by its scramjet engine, the X-51A will accelerate to approximately Mach 6 as it climbs to nearly 70,000 feet. Hypersonic combustion generates intense heat so routing of the engine's own JP-7 fuel will serve to both cool the engine and heat the fuel to optimum operating temperature for combustion. The fuel load and flight profile provides for a 240-second engine burn, transmitting vast amounts of telemetry data on its systems to orbiting aircraft and ground stations, before the vehicle exhausts its fuel supply, splashes down into the Pacific and is destroyed, as planned. Flight test vehicles are not recovered.

The X-51A development team elected from the outset not to build recovery systems in the flight test vehicles, in an effort to control costs and focus funding on the vehicle's fuel-cooled scramjet engine. A U.S. Navy P-3 Orion aids in transmitting telemetry data to engineers at both Naval Air Station Point Mugu and Vandenberg AFB, Calif., before it arrives at its final destination, the Ridley Mission Control Center at Edwards AFB.

Conceived in 2004, the X-51A made its first "captive carry" flight on December 9, 2009. The flight test verified the B-52's high-altitude performance and handling qualities with the X-51 attached and tested communications and telemetry systems, but the vehicle remained attached to the B-52s wing. 

The X-51A made history during its first supersonic combustion ramjet-powered hypersonic flight May 26, 2010, off the southern California Pacific coast. Officials said the flight test vehicle flew as anticipated for nearly 200 seconds, with the scramjet accelerating the vehicle to approximately Mach 5, nearly 3,400 miles per hour. The fuel-cooled scramjet performed as planned transmitting normal telemetry for more than 140 seconds, then observing a decrease in thrust and acceleration for another 30 seconds. An anomaly then resulted in a loss of telemetry, and the test was terminated and vehicle was destroyed by flight controllers on command. 

Despite the anomaly, the May 26 flight is considered the first use of a practical hydrocarbon fueled scramjet in flight. The longest previous hypersonic scramjet flight test performed by a NASA X-43 in 2004 was faster, but lasted only about 12 seconds and used less logistically supportable hydrogen fuel.

Following an extensive analysis of flight data from the X-51A's first hypersonic flight test, slight modifications are planned to strengthen the rear seal area near the engine exhaust nozzles for the three remaining X-51As. Flight tests were resumed in 2011.




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